ATHENS—The risk of deaths among refugees and other migrants crossing into Greece by sea from Turkey, a key passageway in their bid to reach Europe, is expected to rise sharply in coming months with the onset of winter, volunteer groups and Greek officials warn.

Greek officials are bracing for more migrants, most of them refugees escaping conflict and violence in Syria and Afghanistan, to cross over into one of the country’s islands, undeterred by the worsening conditions.